tests/test-abort-checkin.t

dirstate-v2: Add a zero-size error type for dirstate v2 parse errors

This error should only happen if Mercurial is buggy or the file is corrupted.
It indicates for example that:

* A part of the file refers to another part, and the byte offset or item count
  would cause reading out of bounds, beyond the end of the file.
* The byte for an entry state has an invalid value

When parsing becomes lazy, many more functions will return a `Result` with
this error. Making it zero-size reduces the work that the `?` operator needs
to do to pass around the error value.

Differential Revision: https://phab.mercurial-scm.org/D10748

  $ cat > abortcommit.py <<EOF
  > from mercurial import error
  > def hook(**args):
  >     raise error.Abort(b"no commits allowed")
  > def reposetup(ui, repo):
  >     repo.ui.setconfig(b"hooks", b"pretxncommit.nocommits", hook)
  > EOF
  $ abspath=`pwd`/abortcommit.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > abortcommit = $abspath
  > EOF

  $ hg init foo
  $ cd foo
  $ echo foo > foo
  $ hg add foo

mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:

  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]
  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]

  $ cd ..